A multi-focus magnification switching lens-barrel which is mounted in a camera in such manner as to move a plurality of lens groups between the position of a short focal distance and the position of a long focal distance, and is provided with a step zoom mechanism for selecting a plurality of focal distances is disclosed in Japanese Laid-Open Patent H9-329733. The multi-focus magnification switching lens-barrel has a fixed barrel inserted in a rotary barrel, and in the fixed barrel are inserted the first moving barrel holding the front group lenses and the second moving barrel holding the rear group lenses.
The rotary barrel is formed with a power varying cam groove for front group lenses and a power varying cam groove for rear group lenses which serve to guide the first and second moving barrels respectively. The fixed barrel is formed with a focusing cam groove having an inflecting portion and a guide groove for guiding the first moving barrel. A cam pin engaging the power varying cam groove for front group lenses and the guide groove is formed in the first moving barrel, and also, a cam pin engaging the power varying cam groove for rear group lenses and the focusing cam groove is formed in the second moving barrel.
Further, there is provided a gear portion at the outer periphery of the rear end of the rotary barrel, and the rotary barrel driven by a motor rotates with its optical axis as rotary axis. As the rotary barrel is rotated, power varying and focusing operations are simultaneously performed.
That is, when the rotary barrel slightly rotates to the extension side, the first moving barrel linearly moves along the guide groove of the fixed barrel. The second moving barrel makes a refractive move at the inflection portion of the focusing cam groove, becoming less in the amount of movement in the direction of optical axis as compared with the first moving barrel. During the time, the lens system remains unchanged in magnification, and focusing is performed in a range from the infinite distance to the closest distance in a state of being wide. In this way, power varying and focusing at each power varying position are performed by using the same driving source in order to realize the reduction of size and cost of the camera.
However, such a multi-focus magnification switching lens-barrel is formed with a power varying cam groove, focusing cam groove, and guide groove in a rotary barrel, and therefore, in case of mounting it in a very small camera built into a portable electronic apparatus such as a portable telephone, it is difficult to form each groove because the rotary barrel is too small.
To solve this problem, it is possible to use a zoom lens device provided with a cam for enabling a very small camera to have a step zoom mechanism. The zoom lens device comprises, as shown in FIG. 15, one motor 1 as a driving source, cam 3 formed by a spiral rib protruded from cylindrical rotary shaft 2, and lens system A arranged on optical axis Z of pickup element 4.
The rotary shaft of motor 1 and the rotary shaft of cam 3 are fitted with gears (hereinafter called “first gear”, “third gear”) 5, 6. The first gear 5 and the third gear 6 engage middle gear (hereinafter called “second gear”) 7, and the rotational force is transmitted in this way, but it is preferable to omit the second gear 7, making the first gear 5 and the third gear 6 engage each other.
And, the outer surface (at the left in the figure) of cam 3 is zoom cam surface 8, and the inner surface (at the right in the figure) thereof is focus cam surface 9. Zoom cam surface 8 is formed with a plurality of flat and slant portions (five portions here) alternately. Focus cam surface 9 is formed on slant portion.
And, lens system A includes first lens group 11, second lens group 12, and third lens group 13 arranged on optical axis Z from inner to outer side in order, and the third lens group 13 is opposed to pickup element 4. The main role of second lens group 12 is to change the focal distance (zooming) of lens system A. The main role of third lens group 13 is to adjust the image forming position of lens system A, that is, to focus (focusing) to pickup element 4.
First lens group 11 is held by an immovable holding frame (not shown). Second lens group 12 and third lens group 13 are respectively held by holding frames 14, 15 which are individually movable in the same direction as optical axis Z. (Hereinafter called “second holding frame” and “third holding frame.”)
Paired guide shafts 16 are piercing the second holding frame 14 and the third holding frame 15, and both holding frames 14, 15 are guided by guide shafts 16 when moving. On one of the guide shafts 16 are wound activating means like a compression spring which serves to fit zoom follower 17 of second holding frame 14 to zoom cam surface 8, and activating means 18 like a compression spring which serves to fit focus follower 19 of third holding frame 15 to focus cam surface 9. Accordingly, cam 3 is always sandwiched between zoom follower 17 and focus follower 19.
The zoom lens device is configured as described above, and the operation of the device is described in the following with reference to FIG. 16 as well. When motor 1 rotates, the driving force is transmitted by first gear 5, second gear 7, and third gear 6 to rotary shaft 2 of cam 3, then cam 3 is rotated. And then, the engaged portion of zoom follower 17 and zoom cam surface 8, and the engaged portion of focus follower 19 and focus cam surface 9 move outwardly or inwardly along the same direction of optical axis Z.
Since zoom can surface 8 is formed with flat and slant portions alternately, zoom follower 17, as shown by characteristic 82 in FIG. 16, does not move outwardly when it is moving on the flat portion, and it moves outwardly only when moving on the slant portion. Accordingly, zoom follower 17 and second lens group 12 move in a step-like fashion, and thereby, the zoom operation is performed.
On the other hand, since focus cam surface 9 is formed with only slant portion, focus follower 19 continuously moves outwardly as shown by characteristic 84 in FIG. 16. Accordingly, even when motor 1 keeps rotating, and cam 3 keeps rotating, zoom follower 17 engages the flat portion of zoom cam surface 8, and third lens group 13 moves while second lens group 12 is in a stop, thereby executing the focus operation. Thus, both of the zoom and focus operations are performed by one motor 1 in this zoom lens device.
In the zoom lens device described above, if the number of steps is increased to cope with various zooms, the number of flat portions of zoom cam surface 8 increases causing the zoom cam 3 to become larger in diameter, and this is disadvantageous to the size reduction. Contrarily, if the number of steps is decreased, it is possible to reduce the size, but the number of zoom patterns is limited, and the value as a zoom lens device is lowered.
Also, the conventional example presupposes the use of a zoom lens system based on optical design such that the movement of focus mechanism nearly follows the movement of zoom mechanism. And generally, the locus of lens movement or zooming, is partially utilized for focusing.
However, the locus of focus movement greatly deviates from the locus that follows the zoom movement. In other words, in the case of a zoom optical system based on optical design such that the locus of focus movement and the locus of zoom movement are highly independent of each other, it is difficult to realize practically appropriate step zoom operation by using the configuration of the conventional example.
In a zoom optical system wherein focus movement is attained by the movement of a lens group closer to the image forming side, the locus of lens movement for focus movement often nearly follows the locus of zoom movement. On the other hand, in a zoom optical system wherein focus movement is attained by the movement of a lens group closer to the object side, the locus of lens movement for focus movement often greatly deviates from that of zoom movement. Accordingly, in the case of the latter zoom optical system, it is difficult to configure a step zoom lens device by using the configuration of the conventional example.
In the latter zoom optical system, the amount of focus movement is often greater as compared with the former, and it has advantages for design such that it is easier to maintain the accuracy of focus even without satisfying as high design accuracy as that of the former. There still exists a problem as mentioned above, it is unable to provide a high-quality zoom lens device by using such a zoom optical system.